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what is the best way to tell if a main vent on a two pipe system is plugged up without burning yourself? just slowly unscrew it till you get steam and no air? i need to go look at a steam system on tuesday where the problem is several radiators on the top floor don't get hot and haven't for about ten years.i am suspecting a plugged main vent or too low of pressure.steam systems are very rare in our area.